The stratosphere is a relatively clear layer of Earth's atmosphere, almost always cloud-free. Almost always. On Monday, researchers at Argentina's San Martín Base spotted a bank of fantastically colored clouds floating in the stratosphere above the Antarctic Peninsula:

"There were the biggest stratospheric clouds we've seen so far this year," says photographer "Marcelo," who is working at the Base this winter. "It was a colorful spectacle to begin the day."These are called "Polar Stratospheric Clouds" (PSCs). They form in the lower stratosphere when temperatures drop to around minus 85º C. This explains why they are rare; even at the poles, such low temperatures are hard to achieve. High-altitude sunlight shining through tiny (~10µm) ice particles in PSCs produce bright iridescent colors by diffraction and interference. Once thought to be mere curiosities, some PSCs are now known to be associated with the destruction of ozone. www.spaceweather.com